The Durham Rule or "product test" was adopted by the United States Court of Appeals for the District of Columbia Circuit in 1954, in the case of Durham v. U.S. (214 F.2d 862), and states that "... an accused is not criminally responsible if his unlawful act was the product of mental disease or defect". Durham was later overturned in the case U.S. v. Brawner, 471 F.2d 969 (1972). After the 1970s, U.S. jurisdictions have tended to not recognize this argument as it places emphasis on "mental disease or defect" and thus on testimony by psychiatrists and is argued to be somewhat ambiguous.
The problem with the 'product test' was that it gave psychiatric and psychological experts too much influence in a decision of insanity and not enough to jurors. Although an expert witness may testify as to his or her opinion in a trial, judges are reluctant to allow it when the opinion goes to the ultimate issue of a case, i.e. when the opinion alone could decide the outcome of a case. The product test asked expert witnesses to use their judgment in determining whether criminal actions were "'the product' of a mental disease or defect." It is the jury's job to decide whether a defendant's actions were the product of his mental disease or defect. The expert witness' job is to determine whether the defendant possesses a mental disease or defect. Further, often conflicting 'expert witnesses' were put on the witness stand by the prosecution and defense to draw the opposite conclusions regarding the cause of an individual's actions.